1. Field of the Invention
This invention relates to a focus detecting device.
2. Description of the Prior Art
There has heretofore been a focus detecting optical device of a single lens reflex camera in which light passed through a phototaking objective lens is directed to the outside of the phototaking light bath by a quick return mirror and after the light directed to the outside of the light path has been imaged, a first and a second image are obtained from this light through a first and a second re-imaging lens. In such device, photoelectric converters are disposed near the focal planes of the first and second re-imaging lenses and whether or not the phototaking objective lens is in in-focus position is detected by way of the movements of the first and second images on the photoelectric converters which occur when the phototaking objective lens is moved in the direction of the optical axis.
In FIGS. 1 and 2 of the accompanying drawings which illustrate the above-described focus detecting device of a single lens reflex camera, L designates a phototaking objective lens, M denotes a quick return mirror, S designates a focusing screen which is transparent in the neighborhood of the optical axis, L1 denotes re-imaging lenses, and P designates photoelectric converters. The light passed through the lens L is directed onto the focusing screen S via the quick return mirror M which is in observation position, but in FIGS. 1 and 2, the light path is developed for the sake of convenience and the lens L, quick return mirror M, re-imaging lenses L1 and photoelectric converters P are all shown as being disposed on a single plane. In this device, the light passed through the phototaking objective lens L is imaged on its focal plane (i.e., on the focusing screen S), passes through a transparent portion formed in the center of the focusing screen S, and is directed through the pair of re-imaging lenses L1 onto the photoelectric converters P corresponding to the re-imaging lenses L1. Since each photoelectric converter P is disposed near the focal plane of each re-imaging lens L1, the light directed onto each photoelectric converter is again imaged on that photoelectric converter. When the phototaking objective lens L is moved in the direction of the optical axis (the direction z), the image formed on each photoelectric converter P moves in the direction x on the surface of this converter P. Detecting means D receives the outputs of the two converters P and detects the in-focus condition of the phototaking objective lens. As the photoelectric converters, use may be made of photodiode arrays or the like.
Description will now be made of conditions with which the focus detecting device must be provided. The focus detecting device must be provided with the condition (1) that it be high in detection accuracy of in-focus and non-in-focus and the condition (2) that it be capable of detecting in-focus even for an object of low brightness.
In the above-described prior art device, the greater the opening angle between the optical axis of the re-imaging lens L1 and the optical axis of the phototaking objective lens L, namely, the angle .theta. in FIG. 1, the higher the accuracy of focus detection. This is because, when the phototaking objective lens L is moved a predetermined amount in the direction of the optical axis, the amounts of movement of the first and second images on the photoelectric converters P are greater as the angle .theta. is greater. Also, as the F-number of the re-imaging lenses L1 is smaller, that is, as .alpha. in FIG. 1 is greater, the lenses are brighter and the outputs of the photoelectric converters P are greater and therefore, focus detection becomes possible even for an object of low brightness.
However, with the above-described prior art device, it is difficult to satisfy both the conditions (1) and (2). That is, in FIGS. 1 and 2, even the light ray l1 incident on the most marginal portion of the re-imaging lens L1 must be one that has left the exit pupil of the phototaking objective lens L and therefore, .alpha., .beta. and .theta. must satisfy the following relation: EQU .beta..gtoreq..alpha./2+.theta.
If, for this reason, .alpha. is selected to a great value to render the re-imaging lenses L1 bright on the basis of the condition (2), as shown in FIG. 1, .theta. will become small and the condition (1) will be sacrificed, because .beta. has been determined by the size of the exit pupil of the objective lens and the distance between the objective lens L and the focusing screen S. If .theta. is selected to a great value on the basis of the condition (1), as shown in FIG. 2, .alpha. will become small and the condition (2) will be sacrificed, because .beta. has again been determined to a predetermined value.